A wind turbine generates electrical energy by converting the energy in the wind. The electrical energy can be supplied to the electric power transmission network. They have conventionally been situated on land. However, the last few years there have been a growth of offshore wind turbines. In addition, the wind turbines are becoming larger, with a rotor of diameter 80 meters and above, which calls for optimisation of the wind turbine, both to minimise the wear of the wind turbine and to maximise the generated electricity.
Optimisation of the wind turbine can for example be optimising the control system. This is important for new wind turbines as well as already operating wind turbines.
It is known in the art that it is possible to optimise the performance of the wind turbine by pitch control of the blades. For example, in WO 2008/041066 A1 disclose an apparatus for controlling a wind turbine having a number of rotor blades comprising a method of moment imbalance compensation. The wind turbine uses a pitch command to control pitch of the rotor blades of the wind turbine. The control first determines and stores a relationship between various values of instantaneous moment and a pitch modulation, which compensates for deviations of the instantaneous moment from a nominal moment value. The control senses an instantaneous moment of the wind turbine resulting in a moment signal. The control uses the moment signal to calculate a blade pitch modulation needed to compensate for the instantaneous moment imbalance. The calculated blade pitch modulation is, combined with the nominal pitch command, determined to control the rotor rpm for example. Finally, the combination is used to control pitch of the rotor blades in order to compensate for the instantaneous moment deviations of the wind turbine.